The present invention relates to an exhaust emission control system for an internal combustion engine, and more particularly to an exhaust emission control system for an internal combustion engine having an exhaust system provided with a catalyst having an oxygen storing ability and/or a nitrogen oxide storing ability.
A three-way catalyst generally used in an exhaust system of an internal combustion engine has an oxygen storing ability in addition to the essential capabilities of the catalyst. Immediately after shifting from a fuel-cut operation for cutting off the supply of fuel to the engine during a normal operation for supplying the fuel to the engine, an original reducing ability of the three-way catalyst is greatly lowered due to the stored oxygen in the catalyst. This problem is conventionally solved by enriching an air-fuel ratio immediately after termination of the fuel-cut operation to thereby quickly remove the oxygen stored in the three-way catalyst within a short period of time.
Further, it is known that an exhaust emission control system including a NOx (nitrogen oxides) catalyst may be applied to an engine designed to frequently perform a lean operation in which the air-fuel ratio is set in a lean region with respect to the stoichiometric air-fuel ratio. The NOx catalyst has a NOx trapping ability for trapping NOx emitted during the lean operation. In this exhaust emission control system, the NOx contained in the exhaust gases during the lean operation is trapped by the NOx catalyst, so that the air-fuel ratio is intermittently enriched and the NOx trapped by the NOx catalyst is reduced.
Regarding the above-mentioned enrichment of the air-fuel ratio (which will be hereinafter referred to as xe2x80x9creduction enrichmentxe2x80x9d, also includes the enrichment for removing the oxygen stored in the three-way catalyst), if the time period of executing the enrichment process is too short, the removal of the oxygen or NOx becomes incomplete. On the other hand, if the time period of executing the enrichment process is too long, the emission of HC and CO increases. Accordingly, a problem may arise in determining how to decide the time period of executing the enrichment process (the end time of the enrichment process).
In a known method, the reduction enrichment process is executed for a predetermined time period. However, it is difficult to set the execution time period to an optimum execution time period which varies with the engine operating conditions. To cope with this problem, there has been proposed a technique such that an oxygen concentration sensor is provided downstream of the catalyst and the reduction enrichment is ended at the time an output from the oxygen concentration sensor has changed to a value indicative of a rich air-fuel ratio with respect to the stoichiometric air-fuel ratio (Japanese Patent No. 2692380).
However, there is a delay time TD from the time when the target air-fuel ratio changes to terminate the enrichment process until the time when the exhaust gases reflecting the changed target air-fuel ratio reaches the catalyst. As a result, the technique described in Japanese Patent No. 2692380 has the following problem. Although the removal of the oxygen or NOx stored in the catalyst is completed at the time the output from the oxygen concentration sensor provided downstream of the catalyst changes to a value indicative of a rich air-fuel ratio, the exhaust gases reflecting the rich air-fuel ratio are still being emitted during the delay time TD, which results in an increase in the emission quantity of HC and CO.
It is accordingly an object of the present invention to provide an exhaust emission control system which can more properly control the time period of executing the reduction enrichment process for removing the oxygen or NOx stored in the catalyst, thereby maintaining good exhaust emission characteristics.
The present invention provides an exhaust emission control system for an internal combustion engine having an exhaust system. The control system includes exhaust gas purifying means, an oxygen concentration sensor, air-fuel ratio control means, predicting means, and determining means. The exhaust gas purifying means is provided in the exhaust system and has at least one of an oxygen storing ability and a nitrogen oxide storing ability. The oxygen concentration sensor is provided downstream of the exhaust gas purifying device. The air-fuel ratio control means enriches an air-fuel ratio of an air-fuel mixture supplied to the engine with respect to a stoichiometric air-fuel ratio, so as to reduce the oxygen or nitrogen oxides stored in the exhaust gas purifying device. The predicting means calculates a predicted value of an output from the oxygen concentration sensor by using a predictor based on the fuzzy logic reasoning. The determining means determines the completion of the reduction of the oxygen or the nitrogen oxides stored in the exhaust gas purifying means according to the predicted value.
With this configuration, a predicted value of the output from the oxygen concentration sensor is calculated by using a predictor based on the fuzzy logic reasoning, and the completion of the reduction of oxygen or nitrogen oxides stored in the exhaust gas purifying means is determined according to the above predicted value. Accordingly, a more precise predicted value of the output from the oxygen concentration sensor can be obtained on the basis of a relatively simple empirical rule, and the completion timing of the reduction of oxygen or nitrogen oxides can be determined slightly earlier than the actual completion timing. By utilizing the result of this determination, the execution of the time period of the reduction enrichment process can be controlled more properly than that in conventional techniques.
Preferably, the air-fuel ratio control means terminates the enrichment of the air-fuel ratio at the time the predicted value has changed from a value indicative of a lean air-fuel ratio with respect to the stoichiometric air-fuel ratio to a value indicative of a rich air-fuel ratio with respect to the stoichiometric air-fuel ratio.
With this configuration, the enrichment of the air-fuel ratio is terminated at the time the predicted value of the oxygen concentration sensor output has changed from the value indicative of a lean air-fuel ratio to the value indicative of a rich air-fuel ratio with respect to the stoichiometric air-fuel ratio. Accordingly, it is possible to avoid a situation where the enrichment execution time period may last too long, which results in an increase in the emission quantity of HC and CO.
Preferably, the air-fuel ratio control means controls the air-fuel ratio to a value near the stoichiometric air-fuel ratio during a predetermined time period after the termination of the enrichment process.
With this configuration, the air-fuel ratio is controlled to a value near the stoichiometric air-fuel ratio during a predetermined time period after the termination of the enrichment process. Accordingly, a small amount of oxygen or NOx remaining in the exhaust gas purifying means at the time of terminating the enrichment process can be sufficiently removed. That is, in come cases, the oxygen or NOx stored in the exhaust gas purifying means cannot be completely removed, but partially remains even after the termination of the enrichment process, depending on the structure of the exhaust gas purifying means. By maintaining the air-fuel ratio at the value near the stoichiometric air-fuel ratio during the predetermined time period after the termination of the enrichment process, the oxygen or NOx can be completely removed.
Preferably, the predicting means may use the output from the oxygen concentration sensor as an input of the predictor in calculating the predicted value.
With this configuration, the oxygen concentration sensor output may be used as the input of the predictor to calculate the predicted value. Accordingly, the configuration of the predictor can be made relatively simple, and human empirical rules can be easily reflected in the membership functions of the fuzzy logic reasoning. As a result, the membership function can be easily set and the prediction accuracy can be improved.
Preferably, the predicting means uses the output from the oxygen concentration sensor and a parameter including a steady-state component and a component indicative of an amount of change in the oxygen concentration sensor output as inputs of the predictor in calculating the predicted value.
With this configuration, the oxygen concentration sensor output and the parameter including a steady-state component and a component indicative of an amount of change in the actual value are used as inputs of the predictor based on the fuzzy logic reasoning. Accordingly, the state where the oxygen concentration sensor output remains at a substantially constant value, or the state where the oxygen concentration sensor output varies largely can be accurately predicted, so that a precise predicted value can be obtained.
Preferably, the predicting means calculates the predicted value using a min-max-barycenter method and a bar-shaped function on the based upon the fuzzy logic reasoning.
With this configuration, the min-max-barycenter method is used for the calculation of the predicted value, and the bar-shaped function is based upon the fuzzy logic reasoning. Accordingly, the operation for the calculation can be simplified and the control can be performed at shorter repetition periods.
Preferably, the air-fuel ratio control means executes the enrichment process of the air-fuel ratio when a fuel-cut operation for cutting off the supply of fuel to the engine is terminated or when a target air-fuel ratio of the air-fuel mixture supplied to the engine is changed from a lean value with respect to the stoichiometric air-fuel ratio to the stoichiometric air-fuel ratio or to a rich value with respect to the stoichiometric air-fuel ratio.